Electrical alternans relate to the differences in electrical potential at corresponding points between alternate heartbeats. T-wave alternans or alternation is a regular or beat-to-beat variation of the ST-segment or T-wave of an electrocardiogram (ECG) which repeats itself every two beats and has been linked to underlying cardiac electrical instability. Typically, by enumerating all consecutive heart beats of a patient, beats with an odd number are referred to as “odd beats” and beats with an even number are referred to as “even beats.” A patient's odd and even heartbeats may exhibit different electrical properties of diagnostic significance which can be detected from an ECG.
The presence of these electrical alternans is significant because patients at increased risk for ventricular arrhythmia's commonly exhibit alternans in the ST-segment and the T-wave of their ECG. Clinicians may therefore use these electrical alternans as a noninvasive marker of vulnerability to ventricular tachyarrhythmias. The term T-wave alternans (TWA) is used broadly to denote electrical alternans such as these. It should be understood that the term encompasses both the alternans of the T-wave segment and the ST-segment of an ECG.
T-wave alternans (TWA) has been demonstrated in many studies as a strong predictor of mortality, independent of left ventricular ejection fraction (LVEF). More specifically, it has become well known that T-wave alternans has predictive value for arrhythmic events such as tachyarrhythmias. Additionally, T-wave alternans has been determined to be an indicator of various forms of disordered ventricular repolarization, including disorders found in patients with cardiomyopathy, mild to moderate heart failure, and congestive heart failure.
Mechanical alternans, also known as mechanical pulse alternans (MPA), relate to the situation where alternating contractions of the heart exhibit alternating values of contraction force or magnitude that cause ejected blood to exhibit similar alternating values of diastolic pressure amplitude. More specifically, the presence of mechanical alternans can be defined by a consistent alternation in peak left ventricular (LV) pressure, or dP/dt, in successive beats.
Visible mechanical alternans have been observed in patients with severe congestive heart failure caused by global left ventricular dysfunction, and is considered to be a terminal sign in this population. Mechanical alternans is characterized by alternating strong and weak beats with a substantially constant beat-to-beat interval. Although its precise origin remains unclear, studies have suggested a link to abnormal intracellular Ca2+ cycling in failing cardiomyocytes. Studies have also shown that prevalence of mechanical alternans increases with exercise and dobutamine loading compared to rest, indicating that mechanical alternan is a rate dependent phenomenon.
Electrical alternans and mechanical alternans, or more generally, myocardial electrical stability and myocardial mechanical stability, are generally monitored separately. Thus, a comprehensive understanding of myocardial stability is not typically achieved.